A. Field of Invention
This invention pertains to a method and apparatus for treating eye disorders associated with imperfections of a patient's eye and its disability to accommodate for near vision. More particularly, the present invention pertains to a method and apparatus for treatment of presbyopia by shaping an annular or extreme peripheral portion of the patient's cornea to increase its refractive power, preferably using automated laser equipment.
B. Description of the Prior Art
Normally, a sharp image of an object is produced by a person's eye when the image is correctly projected on the retina. The process of focusing the image on the retina is referred to as accommodation, and it describes varying the curvature of the lens to change its focal point. More specifically, objects disposed at distances exceeding a certain threshold (usually about 5 m) are seen clearly by a human eye with no accommodation required and the eye is relaxed. For objects closer than this threshold, the eye must accommodate by squeezing the lens to increase its thickness and change its focal point.
As a person gets older, the lens in his eye and its supporting structure, such as the ligaments or zonules lose elasticity and he slowly loses his ability to accommodate. As a result he can no longer see close objects clearly, i.e., he suffers near vision deficiency. This condition is known as presbyopia.
Until about ten years ago the conventional means of treating presbyopia was by use of positive lenses (in the form of eye glasses or contact lenses). Persons who also had other problems, such as myopia or astigmatism, used negative and cylindrical lenses, respectively. These people had to wear bifocal glasses or contact lenses, i.e., lenses with at least two different portions: a superior portion with one curvature and an inferior portion with another curvature. A person wearing these kinds of lenses has to get used to looking at far objects through the superior portion and looking at close objects (in order of 40 cm or less) through the inferior portion.
Eyeglasses are also known with lenses which change gradually from one portion to another so that the lenses have various zones, each zone being optimized for looking at objects within a particular distance range.
Wearing bifocal or multifocal glasses has several disadvantages. One disadvantage is that some persons can get dizzy from such glasses and in fact they can never get used to them. These people normally have two kinds of glasses: one for near vision and another for distant vision. Another disadvantage is that many people find glasses cosmetically unacceptable.
Contact lenses are generally more acceptable cosmetically then glasses. However it is difficult to make bi- or multifocal contact lenses and so at present as presbyopia sets in, some people with contact lens must resort to glasses as well for near vision.
Recently, new techniques have been developed that use lasers to change the optical characteristics of the cornea. Typically, these methods consist of reshaping the cornea by steepening portions thereof. Some methods and apparatus for performing laser surgery on the eyes are disclosed for example in U.S. Pat. Nos. 5,350,373; 5,425,727 6,129,722 and PCT Publication WO 00/27324 all incorporated herein by reference. All these reference disclose methods and apparatus for corrective eye surgery, such as presbyopia, in which a laser beam is directed at the cornea and an ablation is performed to remove material from the cornea thereby changing its optical transmission characteristics. These procedures are performed using one of two techniques. The first technique involves producing an ablation of the cornea in a central zone thereof. The central zone has a diameter in the range of 1.0–3.0 mm and the ablation causes the central zone to steepen thereby increasing its refractive power. This technique is based on the underlying theory that the central zone of the eye is used for close vision while a peripheral zone of the cornea is used for distant vision. This theory is attributed to the fact that the pupil of the eye is closed by a sympathetic reflex when the person looks at objects located closer than 40 cm. According to this theory, since the pupil opens or dilates for distant objects, the annular portion of the cornea must be used to see far objects.
In other words, some present laser surgical techniques are based on a theory that categorizes the cornea into two zones: a central zone of about 1.0–3.0 mm that is used for near vision (for objects up to 40 cm); and an annular zone extending from 3.0 mm that is used for distant vision. Based on this theory, for each type of vision problem, the eye of a person is corrected by ablating the appropriate zone without modifying the other zone. More specifically, according to this theory, presbyopia is treated by steeping only the zone extending from 1.0 to 3 mm of the cornea to augment the convergence power of this zone, thereby focusing close objects onto the fovea.
According to the second theory the mulitfocality of the central zone of the cornea is used to view objects at different distances. Accordingly, presbyopia can be corrected by partitioning the central zone of the eye into several regions, ablating these regions independently to obtain different curvatures, each curvature defining a different dioptic powers for the respective region. The multifocality thus obtained may be achieved by excimer laser ablation with tissue being removed from the central zone of the cornea at different depths for each optical region. A person can then use each of the regions to look at objects at corresponding ranges including near vision.